1. Field of the Invention
The present invention relates to a friction stir welding machine and a friction stir welding tool, which can be appropriately utilized for welding materials to be welded together, the mater comprising steel materials, such as steel materials to be used for general structures, those for constructional structures and those for steel plates.
2. Description of the Related Art
A typical example of the related art is described in JP-A-2004-082144. This related art teaches a welding tool comprising a pin portion adapted to be in contact with a material to be welded is formed integrally on a central axis of a base material consisting of a heat resistive alloy, wherein lowering of hardness in a higher temperature region can be controlled as well as degradation of strength against friction to the material to be welded, such as a steel material, can be reduced by covering the outer periphery including the pin portion with a coating comprising a ceramic of the silicon nitride (Si3N4) type.
In such a related art, the steel material which is provided as SS400 in JIS (the Japanese Industrial Standard) is used as the material to be welded, and the base material of the welding tool comprises a heat resistive alloy mainly consisting of at least one of Fe, Ni, Co and W.
Other related arts are described in JP-A-2000-301363 and JP-A-248583, for example. In these related arts, improvement against insufficient welding is proposed, which comprises spraying an inert gas, such as N2, He or Ar, in the vicinity of a welding portion (i.e., a portion to be welded) of materials to be welded.
Still another related art is described in JP-A-2003-326372. In this related art, lengthening of the life of a welding tool is proposed, wherein a coating comprising diamond is formed on a surface of a base material of the welding tool so as to prevent components of a material to be welded, comprising an aluminum alloy, a magnesium alloy, a copper alloy or the like, from being welded or alloyed together with the welding tool.
In the related art described in JP-A-2004-082144, since a steel material which has a melting point higher than that of aluminum alloys is used as an object to be welded with friction stir, a welding tool on which a coating comprising a silicon nitride or the like is formed should be used. Such a welding tool may tend to generate chemical decomposition of a material constituting the welding tool and/or diffusion of its constructional elements into the material to be welded due to heat generation caused by friction to the material to be welded, thus causing significant wearing at the working site, such as a shoulder portion or pin portion of the welding tool, which is to be used for generating driving force of stir by contacting with the materials to be welded.
In the case of using a hard metal, such as tungsten carbide, as the base material of the welding tool, the standard energy to be produced from the material at 100° C. is relatively high, i.e., approximately −10 kcal/g·atom. Therefore, the material may tend to be decomposed under a higher temperature environment, thus being thermally unstable. In addition, such a material is likely to cause solid solution of tungsten into the steel material. Thus, wearing of the welding tool to be caused by such phenomena deteriorates the mechanical integrity of the region to be used for friction stir, leading to degradation of the strength of welding. Furthermore, since the life of such an expensive welding tool is ended only by a slight use, there is also a problem from the economical view point.
In the friction stir welding operation for a steel material, the rotational speed of the welding tool is often 250 to 1500 rpm. If employing a rotational speed higher than 1500 rpm, the amount of heat generation can be increased and the materials to be welded can be heated more rapidly, thus reducing the time required for welding as well as enhancing the strength of welding. However, wearing of the welding tool is accelerated. Moreover, in view of the life of such an expensive welding tool, there is a significant limit to be considered in increasing the rotational speed. Therefore, the reduction of the time required for welding and enhancement of the strength of welding are restricted in employing the aforementioned welding tool.
In each of the related arts described in JP-A-2000-301363 and JP-A-2002-248583, control of production of oxides is proposed, in which an inert gas is sprayed in the vicinity of a welding portion of a material to be welded to improve insufficient welding. However, in the case where the material to be welded is a steel material, there is a problem similar to that in the related art in JP-A-2004-082144 described above.
In the related art of JP-A-2003-326372 described above, lengthening of the life of a welding tool is proposed, wherein a coating comprising diamond is formed on a surface of a base material of the welding tool so as to prevent components of a material to be welded from being welded or alloyed together with the welding tool. However, also in this related art, when a steel material is used as the material to be welded, carbon which is a constructional element of the coating reacts chemically with the steel material, resulting in a similar problem as that described in each related art of the aforementioned JP-A-2000-301363 and JP-A-2002-248583.